1、under control, under the control of, in control (of)
under control意指"在控制下，被控制住"，是一表示被動意義的介詞短語，通常用作連繫動詞be的表語。如果表示"在某人的控制下"，亦可用under one's control來表示。under control現在常常引申為"情況正常，狀況良好"的意義。例如：the fire which threatened part of the business district this morning is now firmly under control.今早危及部分商業區的這場大火，現在已被控制住了。i noticed that his legs are not well under control.我發現他的兩條腿不聽使喚。my oldest son is no longer under my control.我的大兒子已不服我的管教了。you can go right back to sleep now, everything's under control.你可以回去睡覺了，一切均已恢復正常。
under the control of亦可解作in the control of,其後通常須接人或部門等名詞，表示"受……控制，受……管轄"的被動意義，它一般用作表語或補語。例如：the university is under the direct control of ministry of education.這所大學直屬教育部門管理。he was in the control of evil men who forced him to do wicked things.他受了壞人的控制，他們強迫他幹壞事。 根據具體語境的不同，
in control既可表示"控制，管理（別人）"的主動含義，亦可表示"受控制"的被動含義。如果表明被控制的對象，其後須接of短語。例如：mr. zhang will be in control when i am out.我外出時由張先生負責。but the next moment he had himself in control.但是他馬上就控制了自己。they were in control of the southern side of the mountain.他們控制著南山坡。
2、under one's arm, in one's arms
這兩個介詞短語中的arm是指"胳臂"，而不是"武器"的意義。注意不得任意改變兩個短語中arm和arms的單複數形式。
under one's arm意為"在某人腋下的"。例如：the man who is carrying a parcel under his arm is my elder brother.（腋下）挾著一隻包裹的那個人是我哥哥。he keeps the dictionary under his arm. 他把詞典挾在腋下。
in one's arms意指"（小孩或東西）被摟在懷裡"。例如：the woman who is coming here has a child in her arms.往這兒來的那位婦女抱著一個孩子。he has the big dictionaries in his arms on his way to the room.在去寢室的途中他懷裡抱著幾本大詞典。
3、under repair, in repair
這兩個介詞短語一般都作表語，補語或後置定語。
under repair屬於表示進行意義的介詞短語，有被動的含義，等於being repaired（在修理中）。例如：the department store is now under repair (= is now being repaired) and it will be reopened on may day.百貨公司正在修繕，將於五一節重新開業。the machine is under repair.那台機器正在修理。
in repair表示某物處在一種"維修良好"的狀態之中，常用in good repair,其反義語為in bad repair或out of repair（失修）。例如：they always keep the road in repair.他們總是把公路保養得很好。the pagoda that was built four hundred years ago, is still in (good) repair.那座四百年前建造的塔現在還完好無損。
4、under the sun, in the sun
under the sun亦可解作beneath the sun。它是個習語，決不可譯作"在太陽下"。它有兩個意思和用法：1.表示地點，意為"在世界上，任何地方"；2.用於加強語氣，常放在疑問詞之後，意為"到底，究竟"，等於on the earth。例如：the president's assassination shocked everyone under the sun.總統遇刺，全世界都為之震驚。in his opinion, california has the best climate under the sun.依他看來，世界上要數加利福尼亞的氣候最好。what under the sun did he do it for?他做那件事到底是為了什麼？
in the sun是"在太陽光下"的意思，指某人或某物處在太陽光的照耀下或之中，因此，它可引申為"曬太陽"，還可轉義為"顯要的，有利的"的意思，因為太陽光耀眼奪目並有益於人類。例如：his character is as fickle as the icicle in the sun.他的性格就象陽光下的水柱一樣說變就變。don't read in the sun.不要在陽光下看書。the cat likes to sit in the sun.貓愛曬太陽。the new generation of writers has achieved a place in the sun.新一代的作家已經功成名就。
5、up and down, ups and downs
up and down是副詞詞組（如果其後接有名詞，則為介詞詞組），只有作狀語，意為"上上下下地，前前後後地，來回地，到處"。它還可引申為"徹底地，完全地，坦率地"的意義。例如：the float bobbed up and down on the river. 筏子在江面載浮載沉。he thrust his hands into his pockets and walked up and down the office with measured steps.他把雙手插在口袋裡，邁著方步在辦公室里踱來踱去。where have you been? i have looked for you up and down for nearly half an hour.你剛才到哪裡去了？我到處找你差不多有半個鐘頭了。here is his hometown which he knows up and down.這兒是他十分熟悉的家鄉。
ups and downs是名詞詞組，可用作主語和賓語，其前可加限定詞my, your, the, this等。它的本意是指"高低，起伏"的意思，但它常用作喻意，表示"盛衰，成敗，苦樂，好壞"等意思。例如：life is full of ups and downs.人生充滿苦和樂。the family suffers those ups and downs to which the great houses of scotland have been ever liable.這個家庭遭遇的興衰，是蘇格蘭許多大家族難以避免的。the fluctuations of market prices, their deviations from values, their ups and downs paralyze and compensate each other.市場價格的上下波動，市場價格的背離價值，市場價格的漲落都是相互抵消和互相補償的。
6、useful for, useful to
這是兩個形容詞詞組，通常用作動詞be或become的表語或後置定語，都表示"對...有用的"的意思，但在用法上有所區別。
useful for中的for表示目的或用途，其後通常接動名詞或事物名詞。例如：these books are useful for writing.這些書對寫作很有用。those old dusters will become useful for polishing the car. 那些舊撣子來撣汽車上的灰倒挺合用的。
useful to中的to表示對象，其後接表示人的名詞或代詞。useful for 有時也可接人的名詞表示對象，這時兩者可以換用。例如：your suggestions have been most useful to us.你的建議對我們極有用處。this grammar book is particularly useful for foreign students.這本語法書對外國學生尤為有用。
7、up, down
up和down既是介詞，又是副詞，它們除了可分別表示"向上"和"向下"外，還有其他的意義。
1). up和down都可表示"沿著"，相當於along；或表示"在遠端"，相當於at the far end of。例如：they are walking up/ down the street hand in hand.他們手牽手沿著街漫步。 can you see the post office up/ down the road?你能不能看到這條路末端的郵電局？
2). up還常用來表示向高處，向重要的地方，向北方，向城市，向安靜的住宅區及大學等。down則常用來表示向低處，向次要的地方，向南方，向鄉村，向熱鬧的商業區及離開大學等。例如：he is flying up to beijing from nanchang.（指向重要的地方。）they went down to the countryside.（指向鄉村。）i wouldn't consider going up north this summer.（指向北方。）she went down town to do some shopping.（指向商業區。）my son will go up to the university next year.（指向大學。）